In plastic substrates used for automobile's bumpers and molds, generally, the wettability of a coating composition on the substrates is poor and the adhesion of a coating composition to the substrates is poor. Particularly when the plastic substrate is a polypropylene resin and the like, since these resins are chemically inactive, the adhesion property of a top coating composition to the substrate are poor. Therefore, it has been commonly implemented to applying a primer before applying the top coating composition. As the primer for this purpose, various solvent type primers have been studied and used, but over the years, water-borne primers using no organic solvent have been proposed from environmental considerations and part of these primers are being used.
The primer is required to provide an excellent adhesion property between a top coating film and a substrate or to impart excellent gasohol resistance to a multilayer coating film formed, but it was generally difficult to achieve these two performance simultaneously by a conventional water-borne primer. And, in recent years, there may be cases where a car is washed by high pressure cleaning, but when an automobile to which the conventional water-borne primer is applied is washed with high pressure jet, a problem that in plastic substrates such as a bumper, a multilayer coating film is peeled off from between the primer and the plastic substrate has arisen. Furthermore, the primer is also required to have moisture resistance, but it was also difficult to achieve the inhibition of peeling of the multilayer coating film in washing a car with a high pressure jet and moisture resistance simultaneously.
As a conventional water-borne primer, a primer including a water-borne chlorinated polyolefin resin, a water-borne alkyd resin and a water-borne novolac epoxy resin is disclosed in Japanese Kokai Publication 2001-139875, and a primer including a water-borne chlorinated polyolefin resin, a water-borne polyurethane resin, a water-borne epoxy resin and an organic strong base and/or salt thereof is disclosed in Japanese Kokai Publication 2004-2801.
But, these primers is required to achieve further improvements in performance of moisture resistance and gasohol resistance and had a problem of peeling of a multilayer coating film in washing a car with a high pressure jet. Further, in recent years, it has been also requested to convert the chlorinated resin to a nonchloric resin since these primers use chlorinated polyolefin resin and there are concerns about produced pollutive gas, such as dioxin in treating the chlorinated resin by burning.
In Japanese Kokai Publication Hei-6-336568, a water-borne primer prepared by adding a hardener such as an oxazolone hardener or an epoxy hardener to a water-borne polyolefin resin, a water-borne acrylic resin and a water-borne polyurethane resin is disclosed, and as a specific example of a primer using a water-borne non-chlorinated polyolefin resin, an example in which maleic anhydride modified ethylene-propylene copolymer is used in conjunction with maleic anhydride modified polypropylene resin to form a water-borne resin is given.
But, when a primer described in this patent was used to form a multilayer coating film on a plastic substrate, it was difficult to achieve the adhesion property to the plastic substrate and the gasohol resistance simultaneously in the obtained multilayer coating film, or to achieve the inhibition of peeling of the multilayer coating film in washing a car with a high pressure jet and the moisture resistance simultaneously.